1. Field of the Invention
For the purposes of describing this invention, the background for this invention, and the examples contained herein, the term "milk" or "fluid milk" means skim milk or nonfat milk (as described in the U.S. Code of Federal Regulations, 21CFR 131), or concentrated skim milk prepared by evaporation or membrane filtration processes, or the fluid prepared from reconstituting powdered skim milk in water, or skim milk to which milk solids have been added for standardization purposes, or skim milk with added whey or whey protein concentrate. Use of this term "milk" is not meant to limit the invention but is meant as a general description of milk protein containing fluids that can be advantageously processed by this invention to produce the products having the beneficial characteristics derived from this invention.
2. The Prior Art
The proteins of milk are valued for nutritional and functional properties when used as ingredients in processed and prepared foods. The proteins are generally categorized into two classes with one class being a heterogeneous mixture called casein and representing approximately 80% of the proteins found in milk, and the second class being a heterogeneous mixture called whey proteins comprising the remaining approximately 20% of the proteins in milk. These proteins are separated from milk using a variety of chemical and physical processing techniques.
Casein is prepared by adjusting the pH of milk to near the isoelectric pH of casein at which pH the casein precipitates and can be collected free of the other soluble components of milk including whey proteins (see Swartz, Encyclopedia of Food Science and Technology, W. H. Hui, editor, Wiley & Sons, 1991, Volume 1, pages 310-318). In a variation of the precipitation technology, Connelly (U.S. Pat. No. 4,376,020) showed that the whey proteins could be made to interact with casein by treating the milk with alkaline and acid pH adjustments. The whey-casein complex thus prepared could be precipitated near the isoelectric pH of casein and the precipitated complex was collected and washed free of other soluble components. Grafferty and Mulvihill (J. Soc. Dairy Technology, 40, 82-85, 1987) reported on the recovery of milk proteins by acid precipitation (pH 4.6) of casein and whey protein complexes formed by heating milk at an alkaline pH. The insoluble acid precipitates are made into functional ingredients for foods by neutralizing with food grade alkaline agents.
Another type of precipitated whey-casein complex is described by DeBoer (U.S. Pat. No. 3,882,256) wherein milk is heated to greater than 90.degree. C. to form the whey-casein complex. Calcium chloride is added to the heated milk to precipitate the whey-casein complex, and the precipitate is washed free of other soluble components from milk. Commercial ingredients made by these precipitation and solubilization technologies are typically called "caseinate", "total milk protein", "milk protein co-precipitate", or "milk protein isolate".
Alternatively, the whey protein and casein protein can simultaneously be separated from the small molecular weight components of milk (lactose, soluble minerals, peptides, nucleic acids, etc.) using porous membrane filters in a process called ultrafiltration (UF). Milk proteins are concentrated by applying pressure to the milk to force water and low molecular weight components through the porous membrane filter while the proteins, fat, and insoluble minerals are retained. Material passing through the membrane is termed the permeate, and material not passing through the membrane is termed retentate. Typically, milk proteins are concentrated by UF to a concentration two- to five-fold over the level in the starting milk. There is a limit to the potential concentration due to the viscosity of the retentate and the dynamics of the membrane filtration process.
Diafiltration (DF) is a similar membrane filtration process wherein water or other diluent is added to the concentrated retentate at or about the same rate that the permeate is removed. Thus, the volume of the retentate may not change much during the process of diafiltration, but the low molecular weight materials are continuously removed from the high molecular weight components in the retentate.
In the process described by Buhler, at al (U.S. Pat. No. 4,125,527) ordinary milk, heated milk, milk plus added whey, and milk adjusted to an acidic pH to enhance the removal of otherwise insoluble calcium salts, were processed by ultrafiltration and diafiltration techniques to concentrate and separate the whey and casein in the retentate using a membrane designed to retain components greater than 10,000 molecular weight.